Chapter 4

1:
A:	A routing protocol is a "language" that routers speak to each other to share information about network destinations.
2:
A:	At a minimum, a routing protocol should define procedures for: Passing reachability information about networks to other routers Receiving reachability information from other routers Determining optimal routes based on the reachability information it has and for recording this information in a route table Reacting to, compensating for, and advertising topology changes in an internetwork
3:
A:	A route metric, also called a route cost or a route distance, is used to determine the best path to a destination. Best is defined by the type of metric used.
4:
A:	Convergence time is the time a group of routers take to complete the exchange of routing information.
5:
A:	Load balancing is the process of sending packets over multiple paths to the same destination. Four types of load balancing are: Equal cost, per packet Equal cost, per destination Unequal cost, per packet Unequal cost, per destination
6:
A:	A distance vector protocol is a routing protocol in which each router calculates routes based on the routes of its neighbors and then passes its routes to other neighbors.
7:
A:	Several problems associated with distance vector protocols are: A susceptibility to incorrect routing information because of its dependence on neighbors for correct information Slow convergence Route loops Counting to infinity
8:
A:	Neighbors are routers connected to the same data link.
9:
A:	Route invalidation timers delete routes from a route table if they exceed a certain age.
10:
A:	Simple split horizon does not send route information back to the source of the route information. Split horizon with poisoned reverse sends the information back to the source but sets the metric to unreachable.
11:
A:	Counting to infinity occurs when routes update a route over a loop; each router increases the metric of the route until the metric reaches infinity. The effects of counting to infinity are controlled by defining infinity as a fairly low metric so that infinity is reached fairly quickly and the route is declared unreachable.
12:
A:	Holddown timers help prevent routing loops. If a route is declared unreachable or if the metric increases beyond a certain threshold, a router will not accept any other information about that route until the holddown timer expires . This approach prevents the router from accepting possibly bad routing information while the internetwork is reconverging.
13:
A:	A distance vector router sends its entire route table, but it only sends the table to directly connected neighbors. A link state router sends only information about its directly connected links, but it floods the information throughout the internetworking area. Distance vector protocols usually use a variant of the Bellman-Ford algorithm to calculate routes, and link state protocols usually use a variant of the Dijkstra algorithm to calculate routes.
14:
A:	A topological database holds the link state information originated by all routers in the link state routing domain.
15:
A:	Each router floods a link state information advertisement describing its links, the states of its links, and any neighboring routers connected to those links, throughout the internetworking area. All routers store all received copies of the link state advertisement in a link state database. Each router calculates a shortest path tree from the information in the topological database and enters routes in its routing tables based on the shortest path tree.
16:
A:	Sequence numbers help a router differentiate between multiple copies of the same link state advertisement and also prevent flooded link state advertisements from circulating endlessly throughout the internetwork.
17:
A:	Aging prevents old, possibly obsolete, link state information from residing in a topological database or from being accepted by a router.
18:
A:	A router builds a shortest path tree by first adding itself as the root. Using the information in the topological database, the router creates a list of all of its directly connected neighbors. The lowest-cost link to a neighbor becomes a branch of the tree, and that router's neighbors are added to the list. The list is checked for duplicate paths, and if they exist, the higher-cost paths are removed from the list. The lowest -cost router on the list is added to the tree, that router's neighbors are added to the list, and the list is again checked for duplicate paths. This process continues until no routers remain on the list.
19:
A:	Within a routing domain, areas are subdomains. They make link state routing more efficient by limiting the size of the link state database of each router in the area.
20:
A:	Depending on the usage, an autonomous system can be defined as an internetwork under a common administrative domain or a single routing domain.
21:
A:	An Interior Gateway Protocol is a routing protocol that routes within an autonomous system. An Exterior Gateway Protocol is a routing protocol that routes between autonomous systems.